Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, three-phase power at high voltage (345,000 volts phase-to-phase) is delivered to multiple transmission substations at which transformers step this high voltage down to a lower three-phase voltage (69,000 volts phase-to-phase). This 69,000 volt three-phase power then feeds multiple distribution substations whose transformers further step down the voltage to the distribution voltage (12,470 volts phase-to-phase) and separate the power into three single-phase feeder cables. Typically, these feeder cables operate at 7,200 volts phase-to-ground. Each of these feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120/240 volts for delivery to commercial and residential customers.
Electrical apparatus associated with the generation, transmission, and distribution of high voltage electric power can produce electrical discharge corona when the surrounding air begins to lose insulating qualities. Discharge corona produced by transmission lines, transformers, switches, insulators, bushings, and the like occur when the air surrounding them begins to conduct rather than insulate. The high local electric field around these devices ionizes the air, causes excitation of nitrogen molecules in the air, and leads to an electrical partial discharge and ultraviolet radiation.
Discharge corona generates ozone and nitrogen oxides, which can form corrosive compounds such as nitric acid. These corrosive compounds can significantly reduce the service life of electrical components, cause damage to high voltage insulators, and create radio interference. Electric utility companies attempt to find and correct discharge corona as early as possible to prevent later failures that can cause a power outage to their customers.
Discharge corona emits radiation within the spectral range of about 280 nm to about 400 nm which falls mostly in the UV range and is therefore invisible to the human eye. The spectrum of discharge corona and visible light is illustrated in FIG. 1. Strong discharge corona can show up very faintly to the human eye in total darkness and can also be detected using standard UV cameras. However, sunlight UV masks discharge corona making it difficult or impossible to visibly see or use standard UV cameras in the daytime.
To detect discharge corona in daylight, all modern corona cameras use what are known as “solar blind” filters. UV energy from the sun, at wavelengths shorter than around 280 nm, is completely absorbed by the atmosphere. Therefore, by using a shortpass or bandpass UV filter, solar energy appears dark to UV cameras while discharge corona energy shorter than 280 nm is passed by the solar blind filter. Therefore, solar blind UV cameras can be used both at night and during daytime.
U.S. Pat. No. 8,781,158 discloses a common implementation of essentially all modern solar blind UV cameras. The basic approach is illustrated in FIG. 2 where a separate optical path is provided for the solar blind spectrum and another separate optical path is provided for the visual spectrum. The images from these separate paths are then combined and displayed to the user. This is required so that the exact position of the corona on the electrical component can be determined.
U.S. Pat. No. 7,732,782 discloses an attempt to reduce the complexity of this dual optical path approach by implementing a single optical path approach as illustrated in FIG. 3 in which a dual narrowband filter is used to pass both discharge corona UV energy and visible spectrum energy.
The problem with all current solar blind approaches is that the discharge corona energy in the solar blind spectrum is minuscule so expensive image intensifiers must be used to detect the corona. A typical intensified charge-coupled device (ICCD) includes a photocathode, micro-channel plate, phosphor screen, and CCD image sensor mounted one close behind the other. When light source photons fall onto the photocathode they generate photoelectrons which are accelerated and multiplied inside the micro-channel plate by an electrical voltage. The phosphor screen converts the multiplied electrons back to photons which are received by a camera CCD image sensor for conversion into a digital image. This is the same principle used by many night vision goggles and cameras.
In addition to the ICCD device, solar blind filters and the merging of the UV and visible images typically require expensive optical components and focus/zoom lenses. The result is that these solar blind corona cameras typically cost on the order of $40,000 USD in 2014. Many small utilities simply cannot justify these costs and forgo the benefits of early discharge corona detection.
Accordingly, it is the object of the present invention to provide a new and improved method and apparatus for discharge corona detection in AC power systems that greatly reduces the cost of a corona camera.
It is another object of the present invention to provide long range discharge corona detection in AC power systems so long spans of transmission and distribution lines can be observed from a central location.
It is another object of the present invention to provide stationary automated or semi-automated substation discharge corona detection in AC power systems using programmed auto-patrol (security camera like) observation.